Equalizing series resistors for parallel circuits in rectifiers



Aug. 21. 1956 R. c. HITCHCOCK 2,760,142

EQUALIZING SERIES RESISTORS FOR PARALLEL cmcums IN RECTIFIERS Filed Aug.25, 1954 2 Sheets-Sheet 1 AC 7 7 /7 Fig.5

P \J v A c 30 a r If INVENTOR. Q1 cHAeo CH/TcHcOcK H15 A 7'T'OEMEY 21.1956 R. c. HlTCHCOCK 2,750,142

EQUALIZING SERIES RESISTORS FOR PARALLEL CIRCUITS IN RECTIFIEIRS FiledAug. 25, 1954 2 Sheets-Sheet 2 QQF M) INVENTOR. ,Q/cHA 20 C. H/TCI-ICOCKH s ATT'OQME'V Fig.4,

United States Patent EQUALIZING SERIES RESISTORS FOR PARALLEL (IIRCUITSIN RECTIFIERS Richard C. Hitchcock, Indiana, Pa., assignor t0 SyntronCompany, Homer City, Pa., a corporation of Delaware Application August25, 1954, Serial No. 452,030

6 Claims. (Cl. 321-27) This invention relates generally to rectifiersand more particularly to equalizing series resistors for use in parallelpaths of connected rectifiers. Rectifiers, and particularly those of thedry disc type, have a forward nonlinear current characteristic. Thisnonlinear variation in voltage drop for different currents in theforward direction varies for different rectifiers of the same type andsize. Some rectifiers may be matched at a specific voltage drop for aspecific current value but a different current value may give anentirely different voltage drop in these rectifiers.

When rectifiers are connected in multiple to increase the rectifiedcurrent rating some of these rectifiers may be evenly matched in theirvoltage drop for a specific current or current range but if the currentload varies from this current or current range, the rectifier having thelowest voltage drop will assume most of the load and eventually willbreak down, causing destruction of substantially the whole circuit.

The principal object of this invention is the provision of an equalizingseries resistor in each multiple path of the parallel connectedrectifiers whether they are tubes, vibrators or dry disc type ofrectifiers to limit the current flow in any one path and thus attainequalization of the current to such a degree that the rectifier of thelowest voltage drop will not assume most of the load and burn out. Bypreventing the failure of one rectifier of a parallel group the circuitand the rectifiers are maintained in service for a materially longerperiod of time.

In the case of a dry disc type of rectifier the whole stack is savedbecause the low voltage drop of only one cell was prevented from takingenough current to burn it out.

In dry disc rectifiers having many multiple paths and a few or no cellsin series this problem is much more serious. The greater the number ofmultiple paths, the more acute the problem.

This is readily solved by inserting a resistance in series with eachmultiple path and thus limiting the final distribution of the current.

Other objects and advantages appear hereinafter in the followingdescription and claims.

The accompanying drawings show for the purpose of exemplificationwithout limiting the invention or claims thereto certain practicalembodiments illustrating the principles of this invention wherein:

Fig. 1 is a diagrammatic View of a plurality of mercury arc rectifiersconnected in multiple and each having an equalizing series resistor.

Fig. 2 is a diagrammatic view of a plurality of diodes connected inmultiple and each has an equalizing series resistor.

Fig. 3 is a diagrammatic view of a plurality of dry disc rectifiersconnected in multiple and each having an equalizing series resistor.

Fig. 4 is a diagrammatic view of a plurality of full wave rectifiers ofthe dry disc type connected in multiple "ice and each leg of eachrectifier has an equalizing series resistor.

Fig. 5 is a diagrammatic view of a plurality of full wave rectifiers ofthe dry disc type connected in multiple and each leg has one or morerectifier cells in series with an equalizing resistor.

Fig. 6 is an actual showing of a plurality of full wave rectifier cellsof the dry disc type stacked and connected in multiple for a three phasesystem and each cell is connected through a series resistance to theirrespective circuit line to limit and to equalize the current flowingtherethrough.

Referring to Fig. l of the drawings the mercury arc rectifiers 1, 2 and3 each have two electrodes 4 and 5 and are connected by the filament 6.Each of the electrodes 4 and 5 are connected in multiple and the totalis connected in series with the load L. The electrode 4 of eachrectifier is provided with a resistor such as indicated at 7. Theseresistors 7 may be as much as fifty per cent of the resistance to theflow of current through the operating rectifier. The filament 6 of eachrectifier provides sufficient heat to vaporize the mercury sufficientlyto carry the are between the electrodes 4 and 5. However, the resistanceof the path through the filament is so much higher than that of theionized vapor between the electrodes that very little current passesthrough the filament when the rectifier is operating. The resistances 7are likewise so much lower than the filament that they are negligiblebut they may range from ten per cent to fifty per cent of the resistanceof the gap which is sufficient to equally distribute the load betweenthe rectifiers 1, 2 and 3.

A similar circuit is shown in Fig. 2 using diodes 8, 9 and 10 eachhaving the usual cathode 11 and the anode 12. In series with each ofthese rectifier circuits a resistor 13 is placed which may be of themagnitude of from ten per cent to fifty per cent of the resistance ofthe average diode of the same size and characteristics. Here again theload L is placed in series with the multiple paths formed between thethree diodes.

In Fig. 3 a group of three dry disc type rectifiers are placed inmultiple as shown at 14, 15 and 16. These may be any type of dry discrectifier such as copper oxide or selenium or any of the other types ofrectifying elements. Each rectifier cell shown in this view is providedwith a series resistor 17 and the Whole of the pulsating current flowfrom an alternating current source passes through the load L to completethe circuit.

In the circuit illustrated in Fig. 4 the group of dry disc rectifiersare connected into three complete full wave bridge circuits 18, 19 and20. These three bridge circuits are in turn connected in parallel.

The direction of current flow or low voltage drop is indicated by thedirection of the arrow and the resistance in this direction must belimited to avoid ununiform distribution of current through theserectifier cells. A limiting resistance 25 is placed in each leg of eachbridge to limit the flow of current therethrough and to equallydistribute this current load. This current limitation may be as low astwenty per cent of the resistance of each cell or as high as fifty percent of the voltage drop in the direction of flow of the high currentthrough the cell. This voltage drop may be in the nature of .35 volt to.50 volt for cells having a voltage range of 1.1 to 1.3 volts whichapproximates i g=26.8% or =45.5%

The load current flowing through these series resistors reduces theavailable voltage for the load and it is this small voltage drop whichis sensitive to changes in for- Patented Aug. 21, 1956'- Each bridgecircuit 1 has a rectifier in each of its four legs 21, 22, 23 and 24..

ward resistance of the cell and thus has the tendency to equalize thecurrent flow between the several cells in parallel. This equalizationminimizes the variations in the cells themselves and properlydistributes the current between the cells regardless of their matching.This distribution remains constant as the lower the cell resistance, thehigher the current and the higher the resistance drop to correct thisfault. Thus the cells do not have the tendency to become overloadedwhich causes them to burn out.

If the cells are stacked with several in series as Well as in parallelthe equalizing series resistor still functions to produce a more uniformdistribution of current in the parallel fields. This is illustrated inFig. 5 Where the full wave rectifiers 26, 27 and 28 are connected withthe series resistance 30 in the same manner as that illustrated in Pig.4 except the cells 31 and 32 in each rectifier leg represent two or morecells in series with each other.

Fig. 6 illustrates the actual rectifier stack as made up ofrectangularly shaped cells approximately twelve bysixteen inches inwhich there are sixty cells to the complete stack. This rectifier stackis assembled for operation on three phase alternating current.

The rectifier cells 31 are single discs and the forward direction ofevery two discs is in the same direction, the o direction of alternatepairs of discs is the same, and the direction of adjacent pairs of discsis opposite. These discs are separated by connection plates or straps 32which may be for the A. C. terminals as well as the plus and minus D. C.terminals. These discs and plates, because of their size, have sixuniformly positioned holes to receive the six stacking bolts 33. Thesebolts are tacked together at one end by a mounting plate 34 having abracket 35 at one end. The bolts are insulated and as the plates arestacked thereon in the proper direction as indicated by the arrows, theyare spaced by current carrying washers 36 but are insulated at theirends by the insulating washers 37.

The rectifier discs 31 as shown have a conducting metal base such asaluminum which is coated with selenium and an oxide is provided on oneface of this disc. The metallic side of the disc has adjacent thereto anordinary conducting washer but the selenium side of the washer has aspecial spring type collecting washer 38. The forward direction of thisselenium cell is from the metallic side of the disc to the oxide sideagainst which the collecting washer engages. Thus these collectingwashers show the direction of the forward current flow to the plus ofthe D. C. It will be seen from Fig. 6 that there is only one cell ineach leg and that the stack ismade up into full wave rectifier bridgecircuits each having four legs with one cell per leg. These are fifteensuch bridge circuits in this single stack and they are all in parallel.

The connection plates or straps 32 on the D. C. side are provided withthe resistors 40 that connect them to the bus lines 41 and 42 of theplus and minus terminals of this rectifier.

On the A. C. side the straps 32 are connected directly 4 to three buslines, A. C. 1, A. C. 2, and A. C. 3 which represent the threealternating current phases of a three phase source.

There being two rows of three assembly bolts 33 to make this stack, itis preferable to employ a connector plate or strap 32 on each row ofbolts for each connection. It is then necessary to connect thecorresponding straps 32 from the same positions. This is done by the busbars 43 on both the A. C. and D. C. sides of the bridge circuits.However, only one resistor 40 is placed in each leg as a multiple ofresistances may create another problem of current unbalanced conditionsin the resistors themselves.

I claim:

1. A rectifier circuit comprising a plurality of rectifiers connected ina closed four legged bridge circuit, a plurality of said bridge circuitsconnected in parallel to produce full wave rectification fromalternating current, and a series resistor in each leg of each parallelpath, said resistors having substantially the same resistance.

2'. A single stack of rectifying cells, a series of connector platesintermediate selected of said rectifier cells, a plurality of resistorseach having one end connected to each of said connectors, andconnections with the other ends of said resistors placing selected ofthe cells in multiple.

3. A full wave rectifier stack comprising a series of groups ofrectifier cells mounted in uniform directional patterns, a series ofconnector plates stacked between the rectifier cells, a resistorconnected at one end to each connector plate, and circuit connectorsattached to the other ends of selected resistors to complete a full wavemultiple path rectifier.

4. The structure of claim 3 characterized in that said resistors are allsubstantially equal to each other.

5. The structure of claim 3 characterized in that said resistors providea voltage drop of from twenty per cent to fifty per cent of the forwardvoltage drop of the rectifier in series therewith.

6. A full wave rectifier stack having alternating current and directcurrent connections comprising a plurality of rectifier cells mounted inparallel full wave four legged bridge connection, connector strapsbetween the cells forming the independent legs of each bridge circuitfor connection with the alternating current and direct currentconnections, and a resistor connected between each direct currentconnector strap of the stack and the direct current connections of thestack, each of said resistors having substantially the same resistance.

References Cited in the file of this patent UNITED STATES PATENTS1,110,550 Hewitt Sept. 15, 1914 1,723,525 Ruben Aug. 6, 1929 2,088,474Haller July 27, 1937 2,188,436 Hugus Jan. 30, 1940 2,444,255 HewlettJune 29, 1948 2,521,687 Cameron et a1. Sept. 12, 1950.

